(A) Field of the Invention
This invention relates to the treatment of waste water having a biological oxygen demand (B.O.D.) to remove the B.O.D. More especially the invention is concerned with a process which permits the employment of a single vessel for carrying out the biological reaction and the secondary clarification by settling of precipitated solids from the biological reaction; still further the invention is concerned with improvements in oxygen dissolving devices which may be employed in the process and apparatus to efficiently dissolve oxygen in the waste water.
(B) Description of the Prior Art
(i) Treatment of waste water
Purification and biological treatment of waste water from municipal and industrial sources prior to discharge into natural water systems conventionally comprises four basic steps carried out in four separate treatment tanks or vessels in series.
A typical treatment plant will comprise a plurality of such series of treatment tanks disposed in parallel to treat water from a common inlet duct and discharge it from a common outlet duct.
By way of example a Municipal treatment plant in Hamilton, Ontario is designed to treat waste water at a rate of 60 million gallons/day; each series of primary clarifier aeration tank and clarification tank treats 7.5 million gallons/day, and there are eight such series in parallel. In the Hamilton plant each aeration tank is 360 ft. long, 60 ft. wide and 15.5 ft. deep, and each clarifier is 120 ft. square and 10 ft. deep; thus each aeration tank has an exposed surface area of 21,600 sq. ft., and each clarifier has an exposed surface area of 14,400 sq. ft.
In such conventional treatment processes the waste water is treated initially in a degritting tank in which the heavy solid particles are permitted to settle out. The water passes from the degritting tank to a primary clarifier which comprises a tank which holds the waste water for a time to permit suspended solid particles to settle out and wherein floating solids and oils and grease are skimmed off. The liquid from the primary clarifier passes to an aeration tank which contains microorganisms for converting dissolved matter in the liquid into insoluble matter, air or oxygen is introduced under agitation into the tank to meet the oxygen requirement of the microorganisms. From the aeration tank liquid containing suspended solids and dissolved matter is passed to a secondary clarifier; clear liquid overflows from the secondary clarifier and solids are removed from a lower portion of the clarifier. A portion of the liquid containing sludge in the secondary clarifier is continuously recycled to the aeration tank for further biological treatment, and the excess is wasted.
Various proposals have been made to modify the conventional treatment apparatus to overcome different disadvantages and to improve the efficiency, for example the modifications described in U.S. Pat. No. 3,476,682, Albersmeyer and U.S. Pat. No. 3,983,031, Kirk.
It has been recognized that as the amount of suspended solids in the liquid entering the secondary clarifier increases, the solids loading becomes the critical factor in design criteria governing the size of the secondary clarifier; and the size of the secondary clarifier increases relative to the size of the aeration tank. Consequently the capital cost of the secondary clarifier represents a major portion of the overall cost. This was discussed in a paper entitled Solids Thickening Limitation and Remedy in Commercial Oxygen Activated Sludge presented by R. E. Speece and Michael J. Humenick of the University of Texas at Austin at the 45th Annual Convention of the Water Pollution Control Federation, Oct. 9, 1972, in Atlanta, Ga.
It was further suggested by Speece and Humenick in the aforementioned paper that it might be possible to meet the problem of secondary clarifier size by reducing the amount of solids being transferred from the aeration tank to the secondary clarifier by employing some solids separation within the aeration tank. Speece and Humenick theorized that the secondary clarifier could perhaps be omitted if a solids separation could be achieved in the aeration tank which reduced the overflow suspended solids down to a level permissible in the final effluent. Speece and Humenick were primarily concerned with an oxygen dissolving device which they called a Downflow Bubble Contact Aerator (DBCA), which they developed, and in particular were concerned with its constructional parameters.
The oxygen dissolving device of Speece is described in U.S. Pat. No. 3,643,403, and Speece has also obtained U.S. Pat. No. 3,804,255 which describes the use of an oxygen contact device in the treatment of waste water.
Nevertheless the disclosures of Speece and Humenick and the U.S. patents of Speece fail to recognize that control of the oxygen added is essential to successful treatment of the waste water in a single vessel.
(ii) Oxygen Dissolving Devices
Gas dissolving devices are known and their function is to introduce and dissolve a gas in a liquid. One such device is described by Speece in the aforementioned U.S. Pat. No. 3,643,403, which is especially concerned with dissolving oxygen in water to aerate the water and increase the dissolved oxygen concentration. The Speece device comprises an upright conical housing through which water is passed downwardly and oxygen is continuously injected into the downwardly flowing water through a bubble disperser located in an upper portion of the conical housing adjacent the water inlet.
The inlet velocity of the water entering the conical housing is designed to exceed the upward buoyant velocity of the gas bubbles. The outlet velocity from the bottom of the skirt of the housing is designed to be less than the upward buoyant velocity of the gas bubbles so that between the inlet and outlet a cloud of bubbles of changing size is held in suspension under highly turbulent conditions.
In the lower portion of the conical housing an equilibrium position is established where the down flow velocity of the water equals the buoyancy of the oxygen bubbles and an oxygen bubble zone is established for prolonged contact with the downwardly flowing water. Speece indicates that eventually the bubbles are displaced from the outlet end of the conical housing by virtue of the continuous injection of bubbles at the bubble disperser causing "crowding" of the bubbles at the lower outlet end.
In U.S. Pat. No. 3,804,255 Speece describes a modification in his oxygen dissolving device in which he includes a bubble harvester in the bubble zone to collect the bubbles, including bubble of waste gases such as nitrogen and carbon dioxide which are continuously stripped from the water, the collected bubbles being vented to atmosphere through a vent tube. Speece indicates that the objective of this is to confine turbulence to the interior of the conical housing of the oxygen dissolving device.
Thus in U.S. Pat. No. 3,804,255 Speece seeks to prevent turbulence externally of the oxygen dissolving device, however, Speece did not recognize that the presence of undissolved oxygen in the biological reaction zone would disrupt the formation of the separate clarified zone. Indeed Speece particularly indicates that the solid separation capability in the waste treatment process and the stability of the interface between the clarified supernatant and the sludge, is preserved by confining turbulence to the interior of the cone member.
Clearly Speece did not recognize the significance of controlling the supply of oxygen introduced into the system so as to meet the biological oxygen demand of the microorganisms and avoid undissolved oxygen in the biological reaction zone. Indeed it is clear that Speece did not contemplate controlling the oxygen supply at all since he included a vent means to avoid build up of excess oxygen and other gases in the bubble zone.
Speece sought to eliminate turbulence in the liquid outside the cone member which he found disrupted the formation of the clarified supernatant layer by agitation of the liquid outside the cone member.
It should be recognized, however, that Speece does not eliminate the presence of gas bubbles outside the cone member. This is because the pressure within the cone member is significantly higher than the pressure outside the cone member. Consequently when oxygenated liquid emerges from the outlet in the cone member of Speece, the release in pressure experienced by the liquid results in evolution of some of the oxygen (dissolved under pressure) with the result that bubbles of oxygen are formed.
The present inventors discovered that the released oxygen, in the form of gas bubbles disturbed the efficiency of the clarifying process by carrying suspended solid particles into the clarification zone; and this was the case even when the emergence of the gas bubbles from solution outside the cone member, did not result in any significant change in the turbulence characteristics of the liquid outside the cone member. In other words, in the sense of Speece, there was substantially no turbulence outside the cone member, but efficient clarification was not obtained because of the bubbles emerging from solution.
An object of this invention is to provide a process of treating waste water biologically in which the biological reaction and secondary clarification of biologically treated water are conducted in a single vessel, thereby permitting considerable economy in plant design.
It is a further object of this invention to provide a process of treating waste water which permits the successful treatment of waste water containing a much higher concentration of waste matter than the conventional process employing a separate aeration tank and secondary clarifier.
It is a still further object of this invention to provide a process of treating waste water in which primary clarification, the biological reaction and secondary clarification are all conducted in a single vessel.